[FFmpeg-devel] [RFC]lavc/mjpegenc_common: Fix aspect ratio

[Moritz Barsnick]

On Mon, Dec 10, 2018 at 02:54:54 +0100, Carl Eugen Hoyos wrote:
> I hope somebody can prove me wrong!

Not sure, see below.

> This would need a slightly more sophisticated update to the decoder.

Because it is also presumed incorrect?

>          put_bits(p, 16, 0x0102);
>          put_bits(p,  8, 0);              /* units type: 0 - aspect ratio */
> -        put_bits(p, 16, sar.num);
>          put_bits(p, 16, sar.den);
> +        put_bits(p, 16, sar.num);
>          put_bits(p, 8, 0); /* thumbnail width */

If a 640x480 (640 x dimension, 480 y dimension) image has a SAR of 4/3,
4 (or the x dimension) is its numerator, 3 (or the y dimension) is its
denominator.

The spec (and Wikipedia) says:
  units (1 byte) Units for the X and Y densities.
    units = 0: no units, X and Y specify the pixel aspect ratio
    units = 1: X and Y are dots per inch
    units = 2: X and Y are dots per cm
  Xdensity (2 bytes) Horizontal pixel density
  Ydensity (2 bytes) Vertical pixel density

So Xdensity and Ydensity should be (e.g.) 4 and 3 in this case, which
are sar.num and sar.den (judging by the struct's field names),
respectively.

It seems to me that that is what the original implementation does.

What is yours and Ulf's interpretation?

Cheers,
Moritz